Magnetic control apparatus



, April 14, 1959 JQRI BURCH ET'AL 2,882,457

LMAGNETIC CONTROL APPARATUS Filed Oct. 24, 1955 2 Sheets-Sheet 1 IN VEN TORS JAMES R. Buacu, P Tuonns H. Li:

ATTORNEY April 14, 1959 J. R. BURCH ETAL MAGNETIC CONTROL APPARATUS 2 Sheets-Sheet 2 Filed Oct. 24, 1955 INVENTORS JAMES R. m THOMATI H- L War 7444!.

ATTORNEY United States Patent MAGNETIC CONTROL APPARATUS James R. Burch, Bloomington, Ill., and Thomas H. Lee, Media, Pa., assignors to General Electric Company, a corporation of New York Application October 24, 1955, Serial No. 542,424

7 Claims. (Cl. 317-156) This invention relates to residually retained electromagnets and control circuits therefor and, more particularly, to control circuits and apparatus for magnetic contactors of the self-retaining type whose energizing coils are designed to carry fairly low currents, for example-4n the neighborhood of one-quarter to four amperes.

Self-retaining electromagnets, such as solenoids or contactors, have their magnetic core and armature constructed so that there is a substantial degree of residual magnetism which retains the armature in its attracted position even after the energizing current through the coil is removed. Such electromagnets have the important advantage that a continuing electrical current need not be supplied in order to retain the magnet in its attracted position against the force of a given mechanical load. The reliability of operation of such self-retained electromagnets has heretofore not been great enough for many commercial applications especially where the electromagnet is designed to have a low-energizing current or requires holding contacts or other interlocks in series with the energizing coil. This is because, for proper operation, it is essential that the current through the energizing coil be interrupted during specific intervals of the alternating-current cycle in the coil in order that a proper degree of residual magnetism will result in the magnetic core and armature.

Accordingly, one object of the invention is to provide a self-retaining electromagnet circuit and apparatus which insures great reliability of operation even with electromagnets having coils designed to use fairly low-energizing currents.

Another object of the invention is to provide a selfretaining electromagnetic apparatus and circuit including an interlock arrangement which insures reliable operation regardless of the type and location of the energizing and deenergizing switches or their duration of closure.

In general, in accord with the invention, a resistor is connected in parallel with the energizing coil of the self-retaining electromagnet which may be a magnetic solenoid, relay or contactor and a small switch is connected in series with the coil. The switch is selected to have a unidirectional current interrupting ability less than the peak current to be passed by the parallel connected coil and resistor, thus assuring that the current will be interrupted only after the coil current peak has passed. This switch of low DC. interrupting capacity may be arranged to be actuated by the movement of the armature itself. thereby insuring proper and reliable selfretention of the armature regardless of the type and location of auxiliary operating means. In order to deenergize the electromagnet, another resistor is connected in series with the energizing coil and another switch to reduce the current in the coil to a level providing insufficient residual magnetism to retain the armature. This second switch may also be arranged to be actuated by movement of the armature itself.

The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, together with further objects and advantages thereof can best be understood by referring to the following description taken in connection with the accompanying drawings in which Figure 1 is a schematic circuit diagram of the magnetic control apparatus embodying the invention.

Figure 2 is a perspective view partly in section of a magnetic contactor such as may be used in the circuit of Figure 1.

Figure 3 is a side View with cover removed of a suitable switch incorporated in the magnetic contactor of Figure 2, and

Figures 4 and 5 are groups of curves illustrating the operation of the magnetic contactor and circuit of Figures 1 and 2.

Referring now to Figure 1, a schematic diagram of one form of the invention is shown as comprising a selfretaining electromagnet 10 having an energizing coil 11, a core 12 and an armature 13 arranged to be attracted toward said core 12 upon energization of coil 11. Core 12 and/or armature 13 are constructed to have suflicient magnetic retentivity to retain a substantial portion of their magnetism when magnetized by the linkage of magnetic flux therethrough from coil 11. Core 12 or arma ture 13 may, for example, be constructed of chrome or high carbon steel for this purpose. The details of the construction of such self-retaining electromagnets are well known and will not be further described here.

In Figure 1, electromagnet 10 is employed to actuate a mechanical load represented by three sets of load current carrying contacts 14, such a device being known to the art as a magnetic contactor. Three conductors 15, 16 and 17 supply three-phase alternating current power through contacts 14 to a suitable electrical load 18, which may, for example, be a three-phase electric motor.

In accord with the invention a resistor 19 is connected in parallel with coil 11 and a small switch 20 is connected in series with coil 11 for connection across a suitable source of alternating current, for example-the single phase of current represented by conductors 16 and 17. An energizing switch 21, which may be a manually-operable push button as shown, is preferably also connected in series with switch 20 and coil 11 and may be remotely located from contactor 16) if desired. Switch 20 is preferably normally closed and actuated by the armature 13 of contactor 10 as indicated by dashed line 13a in order to provide an automatic interlock for the contactor. Where no such interlock is desired, switch 20 may be normally open and manually actuated, and energizing switch 21 omitted from the circuit.

In order to demagnetize and thereby deenergize electromagnet 10, coil 11 is also connected in series with a resistor 22 and a small normally open switch 23 across a suitable source of alternating current which may, for example, be the same phase of the three-phase source represented by conductors 16 and 17 as shown. Another manually operable normally open switch 24 is preferably also included in this deenergizing circuit. Switch 23 is preferably also arranged to be actuated by armature 13 of electromagnet 10 in order to provide a suitable interlock. However, where no such interlock is desired switch 23 may be manually operated and switch 24 eliminated from the circuit.

Referring now to Figure 2, we have shown a magnetic control device incorporating the components of the cir cuit of Figure 1 enclosed within dashed line 25; the components being identified by the same reference numerals. Switches 20 and 23 are small snap-acting switches supported alongside main power contacts 14 in a manner to :be actuated by the cross bar 26 attached to armature 13.

Switches 20'and- 23 are preferably of a conventional single-pole, snap-acting type. Details of one suitable construction for these switches 20, 23 are shown in Figure 3. The actuating buttons 27 of switches 20 and 23 are engaged by actuating members 26a on cross bar 26 during the final portion of its movement and considerably after the main contacts 14 are closed thereby. It is important that switch 20 be selected to have a low direct-current interrupting capacity and preferably the lowest consistent with reliable interruption of the current in its intended circuit. For this reason, switch 20 is preferably of the single-break type with a small contact separation in the neighborhood of 0.010 inch. In accord with the invention, resistor 19 is selected to pass sufficient current that the unidirectional current-interrupting capacity of switch 20 is considerably less than the total current flowing therethrough. It will be appreciated that the small amount of current flowing through the energizing coils of many commercially available contactors makes such a relationship 25 between the switch interrupting capacity and the coil current virtually impossible without the presence of parallel-connected resistor 19.

As shown in Figure 2, resistor 19 may be conveniently connected across terminals 28 of coil 11 of contactor 10 25 while resistor 22 may be conveniently connected between the terminals of switches 20 and 23 that are connected to the movable contacts therein. Suitable internal interconnection may then be made in accord with the schematic circuit diagram of Figure 1.

In the operation of the circuit of Figure 1, push button switch 21 is closed thereby permitting current to flow through normally closed switch 20 and the parallel-com nected coil 11 and resistor 19. The current through coil 11 energizes electromagnet 10 and causes armature 13 to move to close main contacts 14 and, during the final portion of its movement, to open switch 20 and close switch 23. As will be more fully explained hereinafter the phase relation between the currents flowing in resistor 19 and coil 11 as well as the relationship between the interrupting capacity of switch 20 and the peak-current of the circuit are such that current is extinguished in coil 11 at a proper point in the alternating current cycle that the magnetic flux in the magnetic material of the electromagnet 10 is at or near its saturation point. The armature 13 thus remains in its attracted or closed position even against the loading force provided by the by point d, will result in a residual magnetism in the core 12 represented by the line ad on the hysteresis loop. Interruption slightly before the peak c and slightly after the reversal point d will also result in fairly high residual magnetism but somewhat less than that represented by line on. Current interruption during the major portion of the rising-current characteristic, however, for example-between points b and c and especially point e, will result in considerably lower residual magnetism. For greatest reliability ofenergization, therefore, it is desirable that the current interruption occur after the peak coil current c and before a change in direction represented by point d occurs.

The manner by which switch 20, resistance 19 and coil 11 function to insure coil-current interruption between points 6 and d of Figure 4 is shown by the curves of Figure 5 in which curve X represents coil current, curve .R represents resistance current and curve S represents the instantaneous. sum of these currents flowing through switch 20. Because of the inductance in coil 11, the coil current X lags the current through resistance 19 with the result that the total switch current S leads the coil current 11. The resistor current R (Fig. 5) leads the coil current X by an appreciable phase angle, preferably by an angle between and 90 electrical degrees. In the illustrated and preferred embodiment of the invention as shown at Fig. 5 this lead angle is about degrees.

The value of resistor 19 is such that the current there- 0 through is of the same order of magnitude as, for example,

springs 30 of main contacts 14 even though no furthetx.

current is flowing in coil 11. It will be appreciated that this discontinuance of the coil current occurs regardless of the duration that push button 21 remains depressed and the energization of electromagnet 10 is thus independent of the operation of push button 21.

In order to deenergize electromagnet 10, push button switch 24 is closed thereby to pass current through interlock switch 23, resistor 22 and coil 11 of a lower magnitude than the previous energizing current through coil 11 thereby to demagnetize or at least to lower the unidirectional magnetization of the magnetic material of electromagnet 10 below a point suflicient to retain the weight of the armature 13 and the loading force of the springs of contact 14. Consequently, armature 13 drops out thereby opening main contacts 14, reclosing interlock switch 20 and opening interlock switch 23.

The operation of switch 20, resistor 19 and coil 11 during the energization of self-retaining electromagnet 10 can be easily understood by referring to Figures 4 and 5. ,-Figure 4 shows the relationship of an alternating-current wave A in coil 11 to the BH or magnetization vs. flux characteristic, normally called the hysteresis loop, of typi- .cal high-retentivity magnetic material employed for the core or armature ofa self-retaining electromagnet or somewhat greater than, the current through coil 11. The

I total resultant peak switch curent S is thus substantially greater than the peak coil current c and changes direction at time y somewhat after the peak 0 and before the reversal time d of the coil current X. The total switch current S, since it leads the coil current X by an appreciable angle (about 45 degrees as shown at Fig. 5) reaches its peak 8' during each rising-current quarter cycle of the coil current, and the instantaneous total current S is continuously greater than the peak instantaneous coil current throughout such quarter cycles. This ensures that the low interrupting capacity switch 20, if opened during any rising-current quarter cycle of the coil current, will arc and continue to carry current. This phase relation also ensures that the instantaneous total current S passes through zero during each quarter cycle of coil current immediately following peak instantaneous coil current, so that the switch 20 will always interrupt the total current, and thus the coil current, during a decreasing-current quarter cycle of coil current. Because of its higher amplitude, the slope of this resultant switch current S across zero current is also considerably steeper than that of the coil current X.

The switch 20 must, as previously stated, have a low unidirectional current interrupting capacity. It will be evident from the foregoing that it is sufficient if the switch 20 is unable to interrupt any instantaneous current as large as the instantaneous total current S which exists during each rising-current quarter cycle of coil current. This can be ensured by providing a switch 20 which has a unidirectional current-interrupting capacity less than, and preferably appreciably less than the peak instantaneous value of coil current. If switch 20 has, for example, a unidirectional current interrupting capacity slightly less than the peak coil current c, switch 20 will function to interrupt the current flowing in the circuit only during the desired portion of the coil-current cycle indicated by the period 'I. If the switch 20 should happen to be opened during any other portion of the coil current cycle, the contacts of switch 20 will continue to sustain an arc until the succeeding period I is reached whereupon the switch will function to interrupt the current. It will be appreciated that without the additional current and phase- ,shift provided by the presence of resistor 19, switch 20 would be able to interrupt the current over a much greater portion of the coil-current cycle indicated by interval z.

In one example of the successful operation of the invention, a solenoid with a chrome-steel armature having a coil rated at 55-volts at 60 cycles, comprising 485 turns of 0.0159 inch wire and having a DC. resistance of 6.6 ohms at ZS-degrees centigrade was connected in series with a small snap-acting switch having a gap separation of 0.010 inch and in parallel with a resistance of 9.2 ohms. The solenoid was operated 20,000 times at 85 percent of its rated voltage with a -pound sealed-in load. No drop out due to low-residual force occurred.

While we have described specific embodiment of the invention many modifications may be made, and we intend by the appended claims to cover all such modifications as fall within the true spirit and scope of the invention.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A self-retaining alternating current electromagnet comprising relatively movable core and armature members of magnetizable material, an inductive energizing coil mounted to magnetize said members, a resistive impedance connected across said coil, and a switch of low unidirectional current interrupting capacity for connecting said coil and impedance in parallel circuit relation to a source of alternating electric current supply, said switch when closed conducting a total current leading the current in said coil in such phase relation that the instantaneous value of total current exceeds the peak instantaneous value of coil current throughout each risingcurrent quarter cycle of coil current and passes through zero during each quarter cycle of coil current following peak instantaneous coil current, the unidirectional current interrupting capacity of said switch being appreciably less than any instantaneous value of said total current during said rising-current quarter cycles, whereby whenever said switch is opened said coil current is interrupted in a quarter cycle following its peak instantaneous value so that a substantially maximum and uniform residual magnetism remains in said core.

2. A self-retaining alternating current electromagnet comprising relatively movable core and armature members of magnetizable material disposed in normally spaced-apart relation, an inductive energizing coil mounted to magnetize said members thereby to attract said armature to said core, a resistive impedance connected across said coil and having an impedance of the same order of magnitude as the coil impedance, and a switch of low unidirectional current interrupting capacity for connecting said coil and impedance in parallel circuit relation to a source of alternating electric current supply, said switch when closed conducting a total current leading the current in said coil in such phase relation that the instantaneous value of total current exceeds the peak instantaneous value of coil current throughout each rising-current quarter cycle of coil current and passes through zero during each quarter cycle of coil current following peak instantaneous coil current, the unidirectional current interrupting capacity of said switch being appreciably less than any instantaneous value of said total current during said rising-current quarter cycles, whereby whenever said switch is opened said coil current is interrupted in a quarter cycle following its peak instantaneous value so that a substantially maximum and uniform residual magnetism remaining in said core holds said armature attracted to said core.

3. A self-retaining alternating current elcctrornagnet comprising relatively movable core and armature members of magnetizable material, an inductive energizing coil mounted to magnetize said members, a resistive impedance connected across said coil and having an impedance of the same order of magnitude as the coil impedance, and a switch of low unidirectional current interrupting capacity for connecting said coil and impedance in parallel circuit relation to a source of alternating electric current supply, said switch when closed conducting a total current leading the current in said coil in such phase relation that the instantaneous value of total current exceeds the peak instantaneous value of coil current throughout each rising-current quarter cycle of coil current and passes through zero during each quarter cycle of coil current following peak instantaneous coil current, the unidirectional current interrupting capacity of said switch being appreciably less than the peak instantaneous value of said coil current, whereby whenever said switch is opened said coil current is interrupted in a quarter cycle following its peak instantaneous value so that substantially maximum and uniform residual magnetism remains in said core.

4. A self-retaining alternating current electromagnet comprising relatively movable core and armature members of magnetizable material, an inductive energizing coil mounted to magnetize said members, a resistive impedance connected across said coil, said impedance being proportioned to conduct alternating current of a predetermined frequency in leading phase relation with alternating current in said coil by more than 45 and less than electrical degrees, and a switch of low unidirectional current interrupting capacity for connecting said coil and impedance in parallel circuit relation to a source of alternating electric current supply, said switch when closed conducting a total current leading the current in said coil in such phase relation that the instantaneous value of total current exceeds the peak instantaneous value of coil current throughout each rising-current quarter cycle of coil current and passes through zero during each quarter cycle of coil current following peak instantaneous coil current, the unidirectional current interrupting capacity of said switch being appreciably less than the instantaneous value of said total current during said rising-current quarter cycles, whereby whenever said switch is opened said coil current is interrupted always in a quarter cycle following its peak instantaneous value so that substantially maximum and uniform residual magnetism remains in said core.

5. A self-retaining alternating current electromagnet comprising relatively movable core and armature members of magnetizable material disposed in normally spaced-apart relation, an inductive energizing coil mounted to magnetize said members thereby to attract said armature to said core, a resistive impedance connected across said coil and having an impedance of the same order of magnitude as the coil impedance, said resistive impedance being proportioned to conduct an alternating current of predetermined frequency in leading phase relation with alternating current in said coil by more than 45 and less than 90 electrical degrees, and a switch of low unidirectional current interrupting capacity for connecting said coil and impedance in parallel circuit relation to a source of alternating electric current supply, said switch when closed conducting a total current leading the current in said coil in such phase relation that the instantaneous value of total current exceeds the peak instantaneous value of coil current throughout each risingcurrent quarter cycle of coil current and passes through zero during each quarter cycle of coil current following peak instantaneous coil current, the unidirectional current interrupting capacity of said switch being appreciably less than the instantaneous value of said total current during said rising-current quarter cycles, whereby whenever said switch is opened said coil current is interrupted always in a quarter cycle following its peak instantaneous value so that substantially maximum and uniform residual magnetism remaining in said core holds said armature attracted to said core.

6. A self-retaining alternating current electromagnet comprising relatively movable core and armature members of magnetizable material, an inductive energizing r coil mounted to magnetize said members and attract said 'armature to said core, a resistive impedance connected of coil current following peak instantaneous coil current, the unidirectional current interrupting capacity of said switch being appreciably less than any instantaneous value "of said total "current during said rising-current quarter cycle, whereby when said switch is opened said coil current is interrupted in a quarter cycle following its peak instantaneous value so that substantially maximum and uniform residual magnetism remains in said core.

7. A self-retaining alternating current electromagnet comprising relatively movable core and armature members of magnetizable material, an inductive energizing coil mounted to magnetize said members thereby to attract said armature to said core, a resistive impedance connected across said coil and having an impedance of the same order of magnitude as the coil impedance, said resistive impedance being proportioned to conduct an alternating current of predetermined frequency in leading phase relation with alternating current in said coil by more than 45 and less than 96 electrical degrees, and a normally closed interlock switch oflow unidirectional current interrupting capacity actuated to open position when said core attracts said armature and when closed connecting said coil and impedance in parallel circuit relation to a source ofalternating electric current supply,

said switch when closed conducting a total current leading the current in said coil in such phase relation that the instantaneous value of total current exceeds the peak instantaneous value of coil current throughout any risingcurrent quarter cycle of coil current and passes through zero during each quarter cycle of coil current following peak instantaneous coil current, the unidirectional current interrupting capacity of said switch being appreciably less than the instantaneous value of said total current during said rising-current quarter cycles, whereby when said switch is opened said, coil current is interrupted always in a quarter cycle following its peakinstantaneous value so that substantially maximum and uniform residual magnetism remains in said core.

References Cited in the file of this patent UNITED STATES PATENTS 2,344,654 Stong Mar. 21, 1944 2,360,750 Wilms Oct. 17, 1944 2,436,339 Taylor Feb. 17, 1948 2,441,984 Armstrong May 25, 1948 OREIGN PATENTS 75,655 Netherlands Aug. 16, 1954 

